A hallmark of immunity is its longevity, manifested by the durability of memory B cells (Bmem ) and plasma cells (PC).Through the analysis and isolation of extremely rare antigen-specific Bmem populations, we have scrutinized their functional responsiveness and their transcriptome. Such genetic analyses and functional studies in vivo have opened up new biological leads into the cells and factors that govern the fate of Bmem. The future five years will be devoted to new exciting initiatives in Bmem biology. Over the last funding period, we have: 1) Defined the role of BCR affinity in B cell fate, 2) Developed new strategies to isolate and study Bmem, 3) Resolved the role of BAFF family members in Bmem and PC survival; 4) Shown that Bmem can be selectively primed relative to PCs, 5) Identified a lineage commitment factor, Id2, for Bmem, 6) Discounted the contribution of TLR stimulation in Bmem persistence and 7) Identified CCR6 as an important chemokine receptor in Bmem migration and function. Given these findings, the Specific Aims for the next funding period are: 1) To define the role of TLR signaling and bystander T cell help in Bmem survival. The prevailing paradigm is that TLR signaling or bystander T cell help sustains Bmem longevity. Our preliminary data shows that TLR agonists in vivo do not induce Bmem cycling, but instead induce terminal Bmem differentiation. Studies are proposed to comprehensively evaluate TLR and T cell signaling on Bmem fate. 2) Continue studies to understand the role of Id2 as a lineage commitment factor in the differentiation of Bmem. Transcriptional profiling has shown that Bmem over-express Id2. Id2 is a nuclear factor that antagonizes transcription. Studies in ld-2-deficient mice (B-restricted) confirm a selective defect in the Bmem compartment. Studies are proposed to fully evaluate the role of Id2 in post-germinal center B cell differentiation and Bmem in mice where Id2 can be conditionally regulated. 3) Resolve the functional significance that Bmem are selectively responsive to CCR6 stimulation. An hypothesis is presented that selective responsiveness to CCR6 may be central to the longevity of Bmem. Our understanding of Bmem and PC biology is central to the development and improvement of vaccines used to enhance the public health. While vaccines are undoubtedly the most successful form of immunotherapy to date, a broader and more molecular understanding of the factors and cells that govern the durability of immunity is well warranted.